Analysis of body volume may be very useful in a body conscious society to determine progress on reducing body fat and tracking cosmetic appearance. Weight measurement does not provide a complete picture of physical status due to the different densities of muscle cells versus fat cells. Two individuals may have the same weight and height, but have vastly different body volume and composition. Specific measurements such as waist circumference are often used as a proxy to determine changes in size of a person and expected fat composition. However, it may be useful and/or beneficial for an individual to be able to measure the entire volume of the body to determine the progress that exercise or diet may have on body shape and volume. A critical aspect of tracking such performance is the ability to easily and quickly get the measurement one is tracking.
A number of techniques are available on the market to help individuals determine their body composition. The simplest method is measurement of the circumference of specific body parts such as the waist or arms. This provides some information, but does not necessarily provide a complete picture of body volume. A more complete measurement of body volume can be achieved by a technique known as hydrostatic weighing. Here, a person is submerged in an enclosed, water-filled container. The volume of the water that is displaced by the body is equal to the volume of the body. The water volume displaced can be measured with simple geometric formulas, if the container is shaped regularly. Although the above technique can be accurate, it requires a person to go underwater, and may be unpleasant and/or burdensome for individuals. In addition, hydrostatic weighing is not a practical alternative for daily monitoring of body volume.
There are also other techniques to determine body fat percentage, but not necessarily body volume. These include skinfold measurement using calipers in which specific skinfold thicknesses are measured. Finally, bioelectric impedance analysis is another method to estimate body fat. This technique uses high frequency electric current through the body to measure its impedance. Using the different electrical impedances of fat tissue and muscle tissue, an estimate of body fat can be obtained. However, this technique is greatly affected by the state of hydration of the person, and generally has limited accuracy.
Use of air pressure and air displacement to measure volume is referred to as plethysmography. This technique has the potential to achieve the accuracy of hydrostatic weighing, without the inconvenience of the subject going into water. Although this general technique to determine volume has been discussed at least from the 1940s, there has yet to be a solution that is simultaneously accurate, fast, simple, and relatively low-cost. An ideal solution would allow person to get a volume measurement in a fashion similar to a weight scale, simply entering a chamber and getting a volume measurement in a matter of seconds. The current state of the art in such equipment (e.g., the BOD POD® body composition tracking system, available from Cosmed USA, Inc., Concord, Calif.) has not achieved this level of simplicity, speed, and cost. A medical assistant is necessary to operate the system while the person is being measured, and the overall measurement process may be relatively time-consuming. In addition, the user generally must wear a very tight fitting bathing suit during the measurement. The cost of such a system is prohibitively high for daily measurements or personal use by the general public.
Many prior attempts to design such a device require moving mechanical pistons. Others require movable diaphragms to modify the volume of one or both chambers. These techniques limit speed and significantly add to the manufacturing complexity of the system. U.S. Pat. No. 5,450,750 to Abler uses the following equation to determine the volume of air in a reference chamber:P1V1/T1=P2(V1+ΔV)/T2   (1)
A single chamber of varying volume is used to calculate the air displacement in the chamber. In formula (1), ΔV is the mechanical volume change of the single chamber. Mechanical components used to make such change-in-volume determinations can be expensive, slow, and unreliable, however.
U.S. Pat. No. 5,105,825 to Dempster requires the use of volume changes to compress and rarify the air and measure the resultant pressure changes across two chambers. Again, physical compression and rarefication of air through a volume change requires the chamber walls to be movable mechanically, as opposed to being fixed and/or rigid.
U.S. Pat. No. 4,144,763 to Vogelman describes two chambers, with a valve connecting the two chambers. However, Vogelman uses a single pressure sensor to determine a pressure change. The result is that the pressure must be equal in both chambers before a second pressure measurement can be made. Vogelman introduces Boyle's gas law, which states that for a given mass of gas, the pressure and volume are inversely proportional. By invoking Boyle's law as the primary equation for the measurement, the measurement depends on a single gas mass and its behavior under volume expansion. The use of a single pressure requires the air pressure to fully equalize before a pressure measurement can be useful. However, considering the flow rates and practical behavior of air valves (e.g., the flow rate is proportional to the difference in pressure between the two chambers), the time necessary to equalize pressure between two chambers can be substantial, causing user inconvenience, and more importantly, introducing significant sources of error to the measurement.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.